Phytopathogenic cyclic glucohexadecaose from an inverting transglycosylase

Xanthomonas species contain numerous notoriously well-known plant pathogens. Among various pathogenic factors, the role of -1,6-cyclized {beta}-1,2-glucohexadecaose (C{beta}G16) produced by Xanthomonas campestris pv. campestris was shown previously to be vital for infecting model organisms Arabidopsis thaliana and Nicotiana benthamiana. However, enzymes responsible for biosynthesising C{beta}G16 are essentially unknown, which limits the generation of agrichemicals that inhibit C{beta}G16 synthesis. In this study, we discovered that OpgD from X. campestris pv. campestris converts linear {beta}-1,2-glucan to C{beta}G16. Structural and functional analyses revealed that OpgD from X. campestris pv. campestris possesses an anomer-inverting transglycosylation mechanism, which is unprecedented among carbohydrate-active enzymes. The discovery of this unprecedented glucan-generating mechanism reveals a new foundation for the enzymatic synthesis of carbohydrates. Furthermore, identifying C{beta}G16 synthase highly conserved in Xanthomonas provides a broadly adaptable drug target for new-genre agrichemicals that overcome antimicrobial-resistant bacterial issues.